A.) Field of Use
The present invention relates to catalysts, systems, and methods that are useful for treating an exhaust gas resulting from combustion of hydrocarbon fuel, and particularly exhaust gas containing nitrogen oxides, such as an exhaust gas produced by diesel engines.
B.) Description of Related Art
The largest portions of most combustion exhaust gases contain relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2); but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of these undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
One of the most burdensome components to remove from a vehicular exhaust gas is NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and/or nitrous oxide (N2O). The reduction of NOx to N2 in a lean burn exhaust gas, such as that created by diesel engines, is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. NOx can be reduced in a diesel exhaust gas, however, by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reducing agent, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate.
Several chemical reactions occur in a selective catalytic reduction (SCR) system using NH3 as reductant, all of which represent desirable reactions which reduce NOx to elemental nitrogen. The dominant reaction mechanism is represented in equation (1).4NO+4NH3+O2→4N2+6H2O  (1)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume NH3. One such non-selective reaction is the complete oxidation of NH3, represented in equation (2).4NH3+5O2→4NO+6H2O  (2)
Furthermore, the reaction of NO2 present in the NOx with NH3 is considered to proceed according to reaction (3).3NO2+4NH3→(7/2)N2+6H2O  (3)
Further, the reaction between NH3 and NO and NO2 is represented by reaction (4):NO+NO2+2NH32N2+3H2O  (4)
Although the reaction rates of the reactions (1), (3) and (4) vary greatly depending on the reaction temperature and the sort of the catalyst used, that of the reaction (4) is, in general, 2 to 10 times as high as those of the reactions (1) and (3).
The application of SCR technology to treat NOx emissions from vehicular IC engines, particularly lean-burn IC engines, is well known. A typical prior art SCR catalyst disclosed for this purpose includes V2O5/WO3 supported on TiO2 (see WO 99/39809). However, in some applications the thermal durability and performance of vanadium-based catalyst may not be acceptable.
One class of SCR catalysts that has been investigated for treating NOx from internal combustion engine exhaust gas is transition metal exchanged zeolites (see WO 99/39809 and U.S. Pat. No. 4,961,917). However, in use, certain aluminosilicate zeolites such as ZSM-5 and beta zeolites have a number of drawbacks. They are susceptible to dealumination during high temperature hydrothermal ageing resulting in a loss of acidity, especially with Cu/beta and Cu/ZSM-5 catalysts; both beta- and ZSM-5-based catalysts are also affected by hydrocarbons which become adsorbed on the catalysts at relatively low temperatures and are oxidised as the temperature of the catalytic system is raised generating a significant exotherm, which can thermally damage the catalyst. This problem is particularly acute in vehicular diesel applications where significant quantities of hydrocarbon can be adsorbed on the catalyst during cold-start. And beta and ZSM-5 zeolites are also prone to coking by hydrocarbons, which reduces catalyst performance. Accordingly, there remains a need for improved catalyst for selective catalytic reduction processes.